The invention relates generally to the field of telecommunications, and more particularly to systems and methods for improving the performance of data transmissions in wireless telecommunications systems.
In a typical wireless voice/data communications system, there are multiple base stations and multiple mobile stations. Each of the base stations has an associated area of coverage. This area is referred to as a sector. When a mobile station is in a base station's sector, the mobile station can communicate with the base station.
The base stations in a wireless telecommunications system are generally positioned so that each base station is associated with a different area of coverage. This is because it generally is not necessary for a mobile station to be able to communicate with more than a single base station. Nevertheless, the sectors associated with the different base stations in the system typically overlap to some extent in order to ensure adequate coverage of the entire area served by the system. Thus, as a mobile station moves throughout the service area (which typically includes many different sectors), it may be in communication with a single base station, or multiple base stations at any given time.
In one type of wireless communication system, both voice and data communications are enabled, although not simultaneously. When voice call is in progress, the voice data is communicated over a “fundamental” channel. The fundamental channel is used both for voice data and for system signaling messages. This is possible because voice data does not use all of the fundamental channel bandwidth. The fundamental channel is established between the mobile station and a set of base stations known as the active set. The same voice data is transmitted to the mobile station by all of the base stations in the active set. The mobile station uses a weighting algorithm based on the signal strengths (the strongest signals are weighted more heavily) to combine the signals. Because of the nature of the voice communications, the transmission of the redundant data does not impair the performance of the system.
When data communications are in progress, the data is carried over the fundamental channel, which is used both to communicate the data and to communicate system signaling messages. When the amount of data being communicated is large, the base station can set up a supplemental channel to carry additional data and to thereby increase the throughput. The supplemental channel cannot carry system signaling messages.
High-speed data communications are somewhat different than voice communications. Because high-speed data communications have more overhead associated with them, and because they use more power then voice communications, the transmission of redundant data from the base stations to the mobile station may degrade the performance and reduce the capacity of the system (the number of users that can be serviced by the respective base stations). This is, in part, the result of the increased interference that results from the transmission of the redundant data.
Because it is desirable to reduce the interference and resource usage associated with redundant data transmissions, the supplemental channel over which these transmissions are carried is not always on. Data transmissions on the supplemental channel are typically assigned a duration (from 20 milliseconds to 5.12 seconds in an IS2000 system). If this duration expires with no data to be transmitted, the supplemental channel is essentially turned off (i.e., signaling will be required to assign a new duration for transmission of further data). This is in contrast to the fundamental channel, over which at least a minimal amount of dummy data is transmitted at all times. Even though the supplemental channel is not always on, however, prior art systems still do not operate efficiently in terms of their usage of resources and generation of interference. It would therefore be desirable to improve these aspects of existing systems.